The present invention relates to an inkjet recording apparatus for providing output images on recording media, which can satisfy a wide range of needs in the printing industry wherein high quality images must be output at a high speed, needs in the printer industry that are based on requirements in offices and personal requirements and, further, needs in consumer product industries wherein inexpensive and versatile output equipment and the like utilizing various types of recording paper for various purposes are required.
As a conventional electrostatic type inkjet recording system, a slit-jet recording system has been disclosed (Susumu Ichinose et al.: xe2x80x9cSlit-Jet Recording Systemxe2x80x9d, Paper at the First Symposium on Non-Impact Printing Techniques, pp. 119-124, 1984). A description will be made on the slit-jet system based on the side view in FIG. 9a and the perspective view in FIG. 9b. 
As shown in FIGS. 9a, 9b, a recording head is constituted by a head portion 911 and a counter electrode portion 912 proposed in a face-to-face relationship with said head portion 911. The head portion 911 is constituted by an ink ejection port 901 formed like a slit, an upper substrate 902 and a lower substrate 903 that form said ink injection port 901, recording electrodes 904 provided on said lower substrate 903 in units of pixels, a control circuit 907 for switching the ejection of ink from the position of each of the recording electrodes 904 based on a recording signal, and a high voltage power supply 906 for supplying a constant voltage pulse to electrodes selected from among said recording electrodes 904, to cause a potential difference between the counter electrode portion 912 and them when the voltage is applied.
The counter electrode portion 912 is constituted by a support body 917 and a common electrode 905 provided on the support body 917 and is provided in a face-to-face relationship with said ink ejection port 901 with a predetermined microscopic gap therebetween and, further, recording paper 910 is inserted in said microscopic gap.
Ink 908 having high resistance is charged in the ink ejection port 901 of said head portion 911, and the head portion 911 and counter electrode portion 912 are driven. Then, as a result of the application of a constant voltage pulse to said recording electrodes 904 as shown in FIG. 9a, charges are supplied from the recording electrodes 904 to the ink in regions where the ink is to be ejected, and an electric field is generated between the recording electrodes 904 and common electrode 905. The ink 908 receives a Coulomb force in the electric field thus generated to be ejected toward the counter electrode portion 912, and flown ink 909 sticks on to and penetrates into recording paper 910 to provide a desired image output.
According to such a conventional slit-jet recording system, it is possible to avoid limitations on resolution placed by nozzles and to facilitate the cleaning of said ink ejection port 901 by replacing nozzles used for inkjet recording with said ink ejection port 901 in the form of an elongate slit.
Further, according to the slit-jet recording system, color output printing can be easily achieved by using a plurality of said recording heads and supplying each of the ink ejection ports 901 with said ink 908 in a different color and driving it based on a recording signal.
However, conventional recording heads and recording apparatuses according to the slit-jet recording system have had the following problems.
(1) The head portion has integrated functions of wiring to recording electrode patterns divided on the basis of recording pixels and to the circuit for controlling and driving said electrodes independently, an ink chamber for storing a predetermined amount of ink and ink supply means such as an ink tank or ink supply path; the head portion has a very complicated structure which reduces the yield of production.
(2) The durability of the head portion is low because the divided recording electrodes at the head portion are always in direct contact with ink to supply electric charges thereto and because the electrodes have such small divisions that they can be easily corroded by reactions such as electrolysis and oxidation.
(3) Since the constant voltage pulse is applied only to recording electrodes which are to cause the ejection of ink during printing, a large potential difference occurs between the recording electrodes which cause the ejection of ink and those which does not cause the ejection of ink. No insulation treatment can be performed on the surface of the recording electrodes because they supply electric charges to ink and, in addition, adjoining recording electrodes are electrically connected through ink. As a result, a voltage drop can occur between the adjoining electrodes, which disables the generation of the potential difference required for the ejection of ink to reduce the selectivity of ink ejecting positions. A possible solution to this is to increase the potential difference supplied between the electrodes in advance. In this case, however, there is a risk of discharge between adjoining recording electrodes or between the recording electrodes and the common electrode on the counter electrode portion.
(4) Since the electric field generated in the recording head is generated by applying a voltage to divided recording electrodes at the head portion and the common electrode at the counter electrode portion, the electrostatic fields in the regions to which the voltage is applied during the operation concentrate at the head portion where the divided thin electrodes are arranged and spread in the form of a plane at the counter electrode. Therefore, while the ink ejecting positions at the ink ejection port in the head portion is accurately positioned, the landing positions of ink flying toward the counter electrode portion are unstable because of the spread of the electric field. This increases the possibility of misalignment of ink dots as an output image for an apparatus, which makes it difficult to provide a high quality output.
(5) Multi-color printing such as color printing can be performed at least by arranging recording heads associated with respective colors and by driving them independently. However, since this necessitates a recording head and a driving circuit for each color and, the cost and size of an apparatus are increased.
(6) When adjoining recording electrodes are simultaneously driven, i.e., when voltages having the same polarity are applied to adjoining recording electrodes, an interaction occurs between ejected ink droplets, which results in a phenomenon wherein recorded pixels are displaced from normal positions to reduce the quality of the image. Possible causes of this include the fact that turbulence occurs at electric fields generated simultaneously between adjoining electrodes toward the counter electrode as a result of significant mutual influence and the electric fields are not concentrated at the counter electrode which is a common electrode and the fact that physical continuation of ink attributable to ejection port formed like a slit has significant influence. Therefore, in order to avoid such an interaction between adjoining electrodes, divided driving methods have conventionally been used in which recording electrodes are sequentially driven at intervals of several lines, which has inevitably resulted in a reduction of recording speed.
(7) The principle behind the ejection of ink during electrostatic type inkjet recording is the fact that ink in the vicinity of said recording electrodes is charged when a voltage is applied between the recording electrodes and common electrodes, and the ink in said region receives a Coulomb force from electric fields generated between both of the electrodes to be ejected toward the counter electrode.
A Coulomb force F that acts when ink is ejected depends on the strength E of the electric fields generated between both of the electrodes and the amount q of the charges at the ink in the head portion as expressed by an equation F=qE. If it is assumed here that the amount q of the charges at the ink is constant, a Coulomb force F varies depending on the strength E of the electric fields generated between both of the electrodes.
During actual printing, however, as a result of the insertion of a recorded medium between both of the electrodes, the strength of the electric fields generated between the surface of the recorded medium and the recording electrodes varies depending on, for example, the characteristics of the recorded medium, e.g., the electrical characteristics such as the dielectric constant and resistance and geometrical characteristics such as the thickness and surface conditions, and this can affect printing.
Especially, in the case of paper which is most generally used as a recorded medium, it is difficult to obtain a stable potential distribution because of irregularities on the surface thereof and variation of the thickness thereof attributable to the fact that it is constituted by organic fiber. In addition, paper is susceptible to environmental factors such as temperature and humidity and is subjected to significant fluctuations of the electrical and geometrical characteristics including, for example, reduction in the volume resistivity and the occurrence of geometrical expansion and contraction and wrinkles as a result of the absorption of moisture. As a result, it is difficult to achieve stable strength of the electric fields.
As apparent from the above, in order to output a high quality image according to the conventional method, it has been necessary to prepare a dedicated recorded medium having stable electrical and geometrical characteristics to suppress the effects as described above, to further control the temperature and humidity in the apparatus and to control factors that contribute to the ejection of ink such as the applied voltage depending on the type of the recorded medium.
(8) When the electrostatic type inkjet recording system as in the example of the prior art is used, the strength E of the electric fields to apply a Coulomb force to charged ink is expressed by E=V/d where d represents the distance between the electrodes and V represents the potential difference. It is therefore important to keep the distance between the electrodes uniform in the longitudinal direction of the recording head and to set it at a predetermined value. During actual printing, since a recorded medium is inserted between both of the electrodes, the recorded medium is polarized by putting the recorded medium in tight contact with the counter electrode portion to be put into contact with the electrodes, and electrical charges having the same polarity as that of the potential applied to the common electrode appear on the surface thereof to generate stable electric fields between the surface of the recorded medium and the recording electrodes of the head portion.
In the case of a normal inkjet recording apparatus, since ink on the image recording side (hereinafter xe2x80x9cfront sidexe2x80x9d) of the recorded medium has not been dried yet immediately after printing, a method is used wherein recorded medium transport means is provided before, i.e., upstream of, the recording head and wherein the front side of the recorded medium is lightly pressed by a roller having a small contact area downstream of the same in order to minimize the contact between the recording head and itself. In the case of the electrostatic type inkjet recording system, however, since the recorded medium must be inserted into the microscopic gap between the head portion and counter electrode portion, such a mechanism of the recorded medium transport means contacts the ink ejection port of the head portion to smear the recorded medium when the recorded medium is deformed and also urges the recorded medium against the counter electrode portion with a small force t o cause an uneven contact state between them. As a result, the distribution of electrical fields becomes unstable to cause deterioration of images. Among methods to avoid this drawback are a method in which the recorded medium to be transported is secured to the counter electrode portion with, for example electrostatic absorption or air absorption means, a method in which the recorded medium to be transported is wound and rotated around a counter electrode portion formed like a drum, and a method in which the recorded medium is chucked and pulled at the end thereof to be transported . This results in a complicated transport mechanism and inevitably leads to an increase in material and manufacturing cost and also to an increase in the size of an apparatus.
A recording head according to the present invention has a configuration including a head portion constituted by an ink ejection port and a common electrode provided in the vicinity of said ink ejection port for supplying electric charges to ink, ink supply means for supplying ink to the ink ejection port through the common electrode in said head portion, a counter electrode portion provided with a microscopic gap from the ink ejection port of said head portion, recording electrodes formed on the surface of said counter electrode as divisions associated with pixels of a recorded image, voltage supply means for applying predetermined voltages between said common electrode and recording electrodes; and driving means for driving the recording electrodes by controlling the voltage applied to each of said recording electrodes in accordance with an image signal independently.
The invention also provides a recording apparatus having a configuration wherein said recording head is used and wherein recording paper transport means for supplying recording paper to the microscopic gap between the ink ejection port provided at said head portion and the counter electrode and for scanning it in synchronism with the driving of said recording head.
Specifically, the head portion of the recording head serves as a common electrode, and divided recorded electrodes are provided at the counter electrode, which improves yield because the structure of the head portion can be simplified to relax limitations on manufacture. Therefore, the maintenance of a recording head can be improved and the manufacturing cost can be reduced.
Since the recording electrodes are provided at the counter electrode, they will not contact ink and also an insulation treatment can be applied on the surface of the recording electrodes, which makes it possible to maintain a high level of insulation between adjoining recording electrodes. This makes it possible to prevent deterioration of the electrodes and discharge between the electrodes, thereby to expand the life of the recording head. In addition, since a great potential difference can be established between recording electrodes that cause the ejection of ink and recording electrodes that do not cause the ejection of ink, selectivity of ink ejecting positions can be stabilized.
Since electric fields concentrate at the recording electrodes on the counter electrode which are located at the end point in the ink ejecting direction, the accuracy of the landing positions of ink and the quality of an output image can be improved.
Furthermore, the simplified structure of the head portion makes it possible to easily configure the ink ejection port with a nozzle-shaped opening which is divided to accommodate each pixel. Since the ink ejection ports can be individually separated one by one in such a structure, physical continuation of ink acts little between adjoining nozzles. It is therefore possible to improve the accuracy of ink landing positions by canceling interactions between adjoining electrodes and to improve the quality of an output image. By suppressing interactions between adjoining electrodes with such a configuration, the recording head can be driven to accommodate one line simultaneously to realized an increase in recording speed.
Color images can be easily output by arranging a plurality of recording heads, driving them independently and scanning the recording paper in synchronism with the driving.
Color images can be output also by providing a single counter electrode portion in face-to-face relationship with a plurality of head portions and performing matrix driving of both of the electrodes. This makes it possible to make an apparatus compact and to reduce the cost of the apparatus.
This invention provides an intermediate transfer medium for receiving a recorded image on the surface of the counter electrode portion facing the head portion of said recording head and also provides retransfer means for causing ink dots ejected by said head portion to stick on to the surface of said intermediate transfer medium temporarily to transfer a desired image and for retransferring said image on to a recorded medium at a subsequent step. As a result, stable printing can be maintained because images are always recorded on the intermediate transfer medium. Further, since retransfer means is provided separately, printing can be carried out regardless of the type of the recorded medium and, as a result, transfer means of the recorded medium can be a simple mechanism which reduces the cost of the apparatus.